A projection-type image display system magnifies and projects an image displayed on an image display element through a projection optical system, by utilizing the image display element as an optical switching element. Examples used as such image display elements include a transmission-type image display element which transmits and projects light, and a reflection-type image display element which reflects and projects light.
In a projection-type image display system using a transmission-type image display element, (1) a path of an illuminating light directed to the image display element, and (2) a path through which a light transmitted through and modulated by the image display element is projected through a projection lens onto a screen, are provided in different regions, respectively. On the other hand, in a projection-type image display system using a reflection-type image display element, parts or wholes of (1) a path of an illuminating light and (2) a path of a reflected light which has been modulated by the image display element are provided in the same region. Therefore, the projection-type image display system using the reflection-type image display element occupies a smaller space.
Further, in the transmission-type image display system, generally each pixel is equipped with a driving transistor. To prevent transistor characteristics from varying due to projection of light on the driving transistors, light blocking layers are laid on the driving transistors and signal lines to the pixels. The light blocking layers unavoidably make a numerical aperture of the pixels smaller. On the other hand, in the reflection-type image display element, generally the reflection surfaces serve as the light blocking layers, resulting in that the numerical aperture of the pixels is greater than that of the transmission type.
As a projection-type image display system using a reflection-type image display element, the Japanese Publication for Laid-Open Patent Application No. 63-39294/1988 (Tokukaisho 63-39294) discloses a video projection device having an optical system which decomposites/composites light from a light source by polarizing the same. Further, for example, the Japanese Publication for Laid-Open Patent Application No. 4-194921/1992 (Tokukaihei 4-194921) discloses a projection-type liquid crystal display (LCD) system using a polymer-diffusion-type liquid crystal, which modulates intensity of illuminating light by varying diffusion of reflected light, while the Japanese Publication for Laid-Open Patent Application No. 7-128664/1995 (Tokukaihei 7-128664) discloses a liquid crystal projector having a substantially identical structure to that of the foregoing projection-type LCD device.
First of all, a video projection device disclosed by Tokukaisho 63-39294 will be explained below.
FIG. 13 is a schematic view illustrating an arrangement of the video projection device. The video projection device is equipped with a light source 21, a collimator lens 22 which collimates light from the light source, a polarizing beam splitter (hereinafter referred to as PBS) 23, a color decompositing prism 24, LCD elements 25, 26, and 27 which display blue, red, and green image components, respectively, reflection mirrors 28, 29, and 30 mounted on the LCD elements 25, 26, and 27, respectively, and a projection lens 32.
Light emitted from the light source 21 goes through the collimator lens 22, thereby becoming a substantially parallel light, and enters the PBS 23, where the light is split into linearly polarized light components directed in two directions orthogonal to each other. The light reflected by the PBS 23 out of lights obtained by splitting enters the color decompositing prism 24.
The color decompositing prism 24 is composed of a first prism 24A, a second prism 24B, and a third prism 24C. The light incident on the color decompositing prism 24 first enters the first prism 24A, where a blue component is separated by a dichroic interference thin film and is guided to the LCD element 25 for displaying an image of the blue component. The light other than the blue component is incident on the second prism 24B, where a red component is separated by a dichroic interference thin film and is guided to the LCD element 26 for displaying an image of the red component. The rest green component light is incident on the third prism 24C, and is incident on the LCD element 27 for displaying an image of the green component.
The respective color component lights incident on the LCD elements 25, 26, and 27 are reflected by the reflection mirrors 28, 29, and 30, respectively, and go through the LCD elements 25, 26, and 27 again, respectively. Here, the color component lights going through the LCD elements 25, 26, and 27 are subject to modulation of polarization directions, in accordance with image signals of the LCD elements 25, 26, and 27, respectively.
The color component lights subject to the modulation of polarization direction are again incident on the color decompositing prism 24, where they are composed. The composed light is incident on the PBS 23, and only the polarized components which are allowed to go through the PBS 23, out of the light subject to the modulation of polarization direction, are projected onto a screen (not shown) by the projection lens 32.
Next, the projection-type LCD device disclosed by Tokukaihei 4-194912 will be explained below.
FIG. 14 is a schematic view illustrating an arrangement of the projection-type LCD device. The projection-type LCD device is equipped with a light source section including a light source 41 and a paraboloidal mirror 42, a lens 43 for converging light from the light source section, a cross dichroic prism 44, reflection/diffusion-type LCD devices 45R, 45G, and 45B for displaying images of red, green, and blue color components, respectively, a convergence lens 46, first and second blocking masks 47 and 48, a reflection mirror 49, a projection lens 50, and a screen 51.
White light emitted from the light source section is converged by the lens 43, and reflected by the reflection mirror 49. Thereafter, the white light goes through the convergence lens 46, thereby being converted to a substantially parallel light, and is incident on the cross dichroic prism 44.
The white light incident on the cross dichroic prism 44 is decomposited into red, green, and blue color components, and the color components are reflected by the reflection/diffusion-type LCD devices 45R, 45G, and 45B, respectively. Here, degrees of diffusion of the lights of the respective color components upon reflection are varied in accordance with image signals of the reflection/diffusion-type LCD devices 45R, 45G, and 45B, respectively.
The lights of the color components reflected by the reflection/diffusion-type LCD devices 45R, 45G, and 45B are composited by the cross dichroic prism 44, and the composited light enters the convergence lens 46. The light entering the convergence lens 46 is converged to the vicinity of an aperture surrounded by an end of the first blocking mask 47 and an end of the reflection mirror 49, and is projected onto the screen 51 through the projection lens 50.
When the projection-type LCD device conducts black display, lights of the color components reflected by the reflection/diffusion-type LCD devices 45R, 45G, and 45B become diffused lights. The diffused lights are hardly converged to the vicinity of the aperture by the convergence lens 46, but are absorbed and blocked by the first and second blocking masks 47 and 48, or alternatively, reflected by the reflection mirror 49 thereby returning to the light source section. As a result, the diffused lights are not projected on the screen 51. In other words, in such a projection-type LCD device, the convergence lens 46, the first blocking mask 47, and the reflection mirror 49 compose a schlieren optical system, so that an image is enlarged and projected on the screen 51. Therefore, to ensure that brightness and darkness in an image are determined in accordance with diffusion of the reflected light which varies according to image signals, schlieren stoppers such as the first and second masks and the reflection mirror 49 are indispensable.
Subsequently, the following description will explain the liquid crystal projector disclosed by Tokukaihei 7-128664.
FIG. 15 is a schematic view illustrating an arrangement of the liquid crystal projector. The liquid crystal projector incorporates a reflection mirror 61, a convergence lens 62, a cross dichroic prism 63, a diffusion-type liquid crystal panel 64G for displaying an images of a green component, and a first blocking mask 65. Incidentally, FIG. 15 is a top view of the liquid crystal projector, and diffusion-type liquid crystal panels 64R and 64B for displaying images of red and blue components are provided on the upper and lower sides of the cross dichroic prism 63, respectively, though not shown in the figure.
An illuminating light from a light source, reflected by the reflection mirror 61, enters the convergence lens 62. Then, the illuminating light enters the cross dichroic prism 63 which is disposed in contact with the convergence lens 62, and is decomposited into color components of red, blue, and green. Then, the color components are made incident on the diffusion-type liquid crystal panels 64R, 64G, and 64B, respectively, and are reflected. Upon reflection of the lights of the color components by the diffusion-type liquid crystal panels 64R, 64G, and 64B, the lights are diffused in the case of the black display. The lights of the color components reflected by the diffusion-type liquid crystal panels 64R, 64G, and 64B are composited by the cross dichroic prism 63, and the composited light is converged to the vicinity of an aperture diaphragm formed between the first blocking mask 65 and the reflection mirror 61 by the convergence lens 62. Thereafter, the light is projected onto a screen, not shown, by a projection lens, not shown.
In the foregoing arrangement, conventionally, unnecessary light UL due to inner surface reflection of the cross dichroic prism 63 or the like proceeds as indicated by a broken line in FIG. 15, in the same direction as the lights reflected by the diffusion-type liquid crystal panels 64R, 64G, and 64B proceed, and hence, even when the black display is conducted, a part of the unnecessary light UL is projected onto the screen through the aperture diaphragm. This causes deterioration in contrast.
In this aspect, in the foregoing liquid crystal projector, the cross dichroic prism 63 is slightly revolved so as to be disposed with an inclination, so that the unnecessary light UL is prevented from going through the aperture diaphragm. With this arrangement, improvement in the contrast ratio is attempted.
In the aforementioned video projection device, a PBS for switcing optical paths depending on the polarization direction is needed as a constituent member. Generally, the PBS is composed of glass blocks, and such a PBS is great in volume and weight, and costs high. Besides, due to birefringence inherent in the glass which the PBS is made of, a polarization direction of incident light is disordered, whereby to precisely switch optical paths is difficult. As a result, quality of projected images degrades, while a contrast ratio of the same lowers.
On the other hand, in respect of the aforementioned projection-type LCD device or liquid crystal projector, when the black display is to be conducted, most of light is diffused and reflected by the LCD element so as not go through the aperture diaphragm, for realization of the black display. However, no matter how the degree of diffusion of the reflected light during the black display period is heightened, it is unavoidable, from structural characteristics of the device, that the unnecessary light more or less passes the aperture diaphragm. Even in the unnecessary light is reduced to only 1 percent of the total quantity of light, the contrast ratio is deteriorated to 100:1, which is far from the level of practical application.
In the case where the aperture diaphragm is made smaller to block as much of the unnecessary light as possible, the unnecessary light is surely reduced. At the same time, however, the light quantity during the white display period is also reduced, and consequently brightness of projected images lowers. In short, as to the arrangement wherein the black display is performed by diffusing reflected light, lowering of the contrast ratio is a theoretically unavoidable problem.
Another cause that lowers the contrast ratio is unnecessary light occurring due to surface reflection, inside reflection, and the like of optical members. Particularly, the surface reflection is a cause of a magnitude to lower the contrast ratio, since the surface reflection causes light reflected to proceed in the same direction as light modulated by the liquid crystal panels proceeds. Since about 1 percent of surface reflection is still unavoidable even with application of surface-reflection-prevention coating thereto, the contrast ratio deteriorates as long as an optical member causing surface reflection exists.
As a countermeasure against the foregoing problem, an optical member such as the cross dichroic prism may be provided at an inclination, like in the aforementioned liquid crystal projector. With such an arrangement, the surface-reflected light can be prevented from passing the aperture diaphragm. If, however, the cross dichroic prism between the projection lens and the liquid crystal panels is provided at an inclination, an aberration occurs. The aberration has a greater magnitude, as the glass block provided at an inclination causes light transmitted therethrough to have a longer optical path. Therefore, in the case where an optical member such as the cross dichroic prism which may make the optical path of light transmitted therethrough longer is provided at an inclination, a great aberration occurs, thereby making it difficult to remove the aberration by appropriately design the projection lens.
Further, regarding the surface reflection by the liquid crystal panel, in the case where the liquid crystal panel is provided at an inclination to prevent the surface reflection, a focal position varies depending on a display area in the liquid crystal panel, and a projected image also becomes deformed to a trapezoid on the screen. Therefore, it is impossible to provide the liquid crystal panel at an inclination, and the surface reflection by the liquid crystal panel is theoretically unavoidable.